(1) Field of the Invention
The present invention relates to a gate valve of the type in which a handle is rotated to move a valve body up or down within a valve chamber and thus to open or shut a passage, more particularly to a gate valve suitable for use in a large diameter passage of liquid transport piping for liquid such as slurry.
(2) Description of the Prior Art
As a gate valve suitable for liquid such as slurry, there is known a gate valve in which a valve body is screwed on a spindle. Rotation of a handle attached to the spindle moves the valve body up or down in a valve chamber to open or shut a passage of liquid transport piping (see, for example, Japanese Unexamined Utility Model Publication (Kokai) No. 58-36682).
In the above known gate valve, the valve body, which is covered with a resilient cover, is spherically shaped at the lower portion thereof and enlarges in diameter toward the upper portion. The valve body has projections at the upper end thereof for engagement with vertical lead grooves provided on the inner wall of the valve chamber, and is moved up and down by rotating the spindle screwed on the valve body.
When this gate valve is used in piping for liquids such as slurry, however, the grooves in the inner wall of the valve chamber eventually become blocked by the solid particles in the slurry. This obstructs the smooth up and down motion of the valve body opening and shutting the passage. The requisite handle torque therefore increases, and valve operation becomes difficult.
Too much blockage of grooves by the solid particles further prevents the valve body from completely descending to shut the passage. As the valve body cannot completely descend to the valve seat of the valve casing, the valve sealing efficiency decreases.
Also, entry of the solid particles into the threaded portion in which the spindle is engaged with the valve body damages the threaded portion.